Superconductors based on the YBa.sub.2 Cu.sub.3 O.sub.7-x system [123 YBCO], where x.ltoreq.0.6, have been known since IBM researchers discovered them in 1986. They are called "high temperature" superconductors because they are superconducting at temperatures well above absolute zero, e.g., at liquid nitrogen temperature (77.degree. K) and higher.
YBa.sub.2 Cu.sub.3 O.sub.7-x crystals can trap magnetic fields, but the flux density is largely dependent on the grain size and the microstructure. The largest trapped magnetic fields require large grain, single crystal specimens, but growing YBa.sub.2 Cu.sub.3 O.sub.7-x crystals larger than 0.5 cm in diameter is difficult. It is especially difficult to make crystals that can achieve a current density (J.sub.c) exceeding 10.sup.3 A/cm.sup.2 in a magnetic field of 1 Tesla or higher at 77.degree. K. Conventionally made YBa.sub.2 Cu.sub.3 O.sub.7-x superconductors tend to exhibit rapid loss of J.sub.c when subjected to magnetic fields of increasing strength.
Several researchers have attempted to increase crystal and grain sizes and improve J.sub.c by a quench-melt-grow method (Mat'l Res., Vol. 7, No. 4, April 1992, pp. 801-807) or by directional solidification (Cryogenics, Vol. 30, January 1990, pp. 5-10). These prior art methods have not achieved a crystal with sufficient magnetic flux trapping ability and current density capacity for practical application in such devices as low loss magnetic bearings, high strength magnetic clamps, or high gain electromagnetic antennas.